America's Favorite Cooking Oil Linked to Weight Gain

A new study suggests that soybean oil, the most commonly used cooking oil in the US, may have a surprising link to weight gain. Researchers at the University of California, Riverside found that mice fed a diet rich in soybean oil gained significant weight, while genetically engineered mice, with a modified liver protein that change how their bodies handled soybean oil's main fat, stayed lean despite consuming the same amount of food. The difference appears to hinge on how the body processes the primary fat, linoleic acid, per Earth.com. In regular mice, linoleic acid was rapidly converted into oxylipins, molecules linked to inflammation and fat buildup, that rose in tandem with weight gain.

The engineered mice, however, produced far fewer oxylipins and maintained steadier blood sugar levels and less fat in the liver. The study, published in the Journal of Lipid Research, highlights that humans, like mice, have natural variation in the enzymes that process linoleic acid, which may explain why some people gain weight more easily on diets high in soybean oil. Notably, the engineered mice showed higher levels of molecules associated with stronger energy use, while regular mice had drops in key energy molecules, potentially leading to more fat storage. The regular mice also had raised cholesterol, though soybean oil contains none.

The findings suggest that the widespread use of soybean oil—now accounting for nearly 10% of daily calories in the US—may be pushing human biology in unexpected directions, per Fox News. "Soybean oil isn't inherently evil," says UCR professor of cell biology Frances Sladek. "But the quantities in which we consume it is triggering pathways our bodies didn't evolve to handle." While human studies have yet to begin, the researchers caution that the long-term health effects of heavy soybean oil consumption deserve greater scrutiny. They're also looking into whether other oils high in linoleic acid, including corn and sunflower oils, trigger similar reactions, per a release.